Seiko Group Corporation (セイコーグループ株式会社, Seikō Gurūpu kabushiki gaisha), commonly known as Seiko ( SAY-koh, Japanese: [seːkoː]), is a Japanese maker of watches, clocks, electronic devices, semiconductors, jewelry, and optical products. Founded in 1881 by Kintarō Hattori in Tokyo, Seiko introduced the world's first commercial quartz wristwatch in 1969.

Seiko is widely known for its wristwatches. Seiko, Citizen and Rolex are the only three watch companies considered to be vertically integrated. Seiko is able to design and develop all the components of a watch, as well as assemble, adjust, inspect and ship them in-house. Seiko's mechanical watches consist of approximately 200 parts, and the company has the technology and production facilities to design and manufacture all of these parts in-house.

The company was incorporated (K. Hattori & Co., Ltd.) in 1917 and renamed Hattori Seiko Co., Ltd. in 1983 and Seiko Corporation in 1990. After reconstructing and creating its operating subsidiaries (such as Seiko Watch Corporation and Seiko Clock Inc.), it became a holding company in 2001 and was renamed Seiko Holdings Corporation on July 1, 2007. Seiko Holdings Corporation was renamed Seiko Group Corporation as of October 1, 2022.

Seiko watches were originally produced by two different Hattori family companies (not subsidiaries of K. Hattori & Co); one was Daini Seikosha Co. (now known as Seiko Instruments Inc., a subsidiary of Seiko Holdings since 2009) and the other was Suwa Seikosha Co. (now known as Seiko Epson Corporation, an independent publicly traded company). Having two companies both producing the same brand of watch enabled Seiko to improve technology through competition and hedge risk. It also reduced risk of production problems, since one company can increase production in the case of decreased production in the other parties. Seiko remains as one of the world's most recognised watchmaking brands.

At present, quartz and Spring Drive watches are manufactured in Shiojiri, Nagano (Epson), and mechanical watches in Shizukuishi, Iwate (Morioka Seiko Instruments). Some of the watches exported abroad, such as the Seiko 5, are also manufactured in Ninohe, Iwate, by Ninohe Tokei Kogyo (established in 1974 as a parts factory for Seiko).